Epicardial leads may be prescribed for the stimulation of the left ventricle, as an alternative to pacing leads implanted via the coronary sinus, whose implantation requires a delicate approach and suffers various drawbacks.
Unlike endocardial leads introduced via a patient's venous network, the implantation of an epicardial lead constitutes a more invasive operation, usually requiring general anesthesia and surgical techniques. A chest surgeon incises the patient's thorax to gain an access to the “pericardial sac” (the pericardium being the fibroserous envelope that surrounds the heart) and the myocardium itself and to fix the epicardial lead on the external wall of the myocardium, by suturing or screwing.
Several types of epicardial leads have been proposed, but they have a number of drawbacks.
One type of commonly-used epicardial leads is a sutured lead. This sutured type of epicardial leads is very stable, but requires a broad access to allow a work area for suturing the lead by a surgeon. In addition, the area of possible implantation is restricted to the vicinity of the incision in the chest.
It is also known that a screw lead is used to secure an epicardial lead. The screwing may be done directly, but the work area is limited in the same manner as a sutured lead. The screwing may also be done using an implantation tool with a handle, a telescopic support tube, and an articulated head on which the screw lead is mounted. In this case, the surgeon may gain an access to an implantation site beyond the incision, but the possible area of implantation is nevertheless limited by the rigidity of the support tube, and the diameter of the support tube (e.g., 40 French) that the surgeon must insert ahead into the curved pericardial space.
Furthermore, the existing screw leads have a low electrical performance (e.g., pacing threshold peaking, low impedance) due to the large size of the screw. The low electrical performance is desirable to withstand the implantation conditions, but it does not provide a satisfactory current density for impedance measurement. In addition, these screws are relatively traumatic to the tissue, and increase a risk of locally creating reactive fibrosis.
It is important, however, from a mechanical point of view, that the screws of the known leads be oversized, to accommodate the high stress on the screw exerted during implantation, with large implantation accessories that offer very little sensitivity to rotation. The lead head is subjected to a high mechanical stress due to its large displacement, combined with a radial traction exerted through the lead body. The mechanical anchorage of the lead head on the myocardium should be able to resist these mechanical constraints.
Regarding being oversized, the diameter of the screw of an epicardial lead is in the order of 4 to 5 mm compared to 1.2 mm for the screw of a typical endocardial lead; the length of the screw is in the order of 5 to 6 mm for an epicardial lead compared to 1.6 to 1.8 mm for a typical endocardial lead; the wire diameter of the epicardial lead is 0.6 mm compared to 0.3 mm of a typical endocardial lead.
Another feature that is common to known epicardial leads is that the lead body, a sheath connecting the lead head to a pacing generator, is connected at a right angle to the lead head. In other words, the axis of the sheath is perpendicular to the axis of the screw. Consequently, the lead system requires a large volume, thus is traumatic in the area of the stimulation site.
In addition, the geometry and configuration of these epicardial leads prevent the surgeon from extracting them by a simple screwing and require a major surgical operation for extraction.
U.S. Published Application No. 2008/0208166 A1 describes a pre-shaped endocardial catheter for delivery with a double curvature forming a heart-wrapping segment that wraps the outer surface of the heart by covering it over most of the epicardium.
Such a catheter, if combined with a screw lead, would limit the screwing capacity as well as the capacity to move over a large area on the surface of the heart. Indeed, the broad curves of the heart-wrapping structure tend to coincide with the external shape of the heart, by the principle of least energy. The result is a given equilibrium position (i.e., at a given implantation site) for a given curvature. Hence, there is a need for several endocardial catheter models with different curvatures if several possible implantation sites are required.